Due to limited operator visibility toward the rear of many vehicles, audible back-up alarms are often provided to warn personnel in the area that the vehicle is moving backward. However, some vehicles, such as hydraulic excavators, have upper units which can be rotated 360.degree. so that the position of the upper unit with respect to the lower unit may be in any orientation such that the operator's visibility toward the direction of machine motion may be limited or obscured. Thus, on those vehicles, it is desirable to have an audible alarm when the vehicle starts to move in either direction. It is also desirable that such an audible alarm be activated automatically immediately upon the vehicle being put in condition for movement in either direction such as when the vehicle brakes are released or the vehicle power train is shifted from the neutral position. Since the area immediately adjacent the vehicle is the primary concern, if the vehicle is travelling in the forward direction with respect to the upper unit, the operator has good visibility and the alarm is not necessary, and it is desirable to provide a means of manually de-activating the alarm. However, once the vehicle is stopped, the alarm controls should be constructed such that they will automatically restore to their ready condition so that the alarm will again be automatically activated without any attention from the operator when the vehicle is conditioned for travel.